The present invention relates to a method and apparatus for providing uniformly distributed filaments from a spun filament bundle. The invention also relates to a process for forming spunbonded nonwoven webs having superior web uniformity and a favorable ratio between machine and cross direction tensile strengths, and the fabric formed by such process.
Nonwoven fabrics can be made from spun filaments by passing freshly extruded filaments on a stream of air through a plurality of filament distribution devices to transport the filaments onto a moving conveyor screen or belt where a nonwoven web is formed as the conveyor moves past the filament distribution devices. The nonwoven web can then be bonded, utilizing any one of a variety of known methods, to produce an integrated web or fabric.
To obtain nonwoven webs of high quality, it is important that the filaments being supplied to the moving conveyor be separated from each other so that they can be uniformly distributed onto the moving conveyor. It is known in the prior art that transporting the spun filaments by a stream of turbulent air will tend to separate the filaments from each other to a certain degree. It is also known that separation of spun filaments can be achieved by application of an electrostatic charge, i.e., a corona discharge, to the filaments prior to supplying the filaments onto the moving conveyor.
One of the problems encountered in forming a nonwoven web from spun filaments is achieving adequate filament separation so that the filaments can be uniformly distributed onto the conveyor. If the filaments laid down to form the web stick together, entangle with other filaments or are laid onto the nonwoven fabric too close to other filaments, the web will have a nonuniform, rope-like appearance which is unsightly and undesirable.
Another problem that is encountered in nonwoven web formation is the interaction between adjacent filament distribution devices which are normally provided in series to form a nonwoven web onto the moving conveyor. These adjacent distribution devices may adversely affect the uniform distribution of filaments because the turbulent flow of air from each device tends to interfere with the flow of air from adjacent devices.
Another problem that arises in utilizing an electrostatic charge to enable filament separation in forming a nonwoven web is providing the electrostatic charge uniformly to each individual filament. If the electrostatic charge is applied to a bundle of filaments, those filaments on the surface of the bundle will receive a substantially greater charge than those filaments at the interior of the bundle. This will result in a nonuniform application of charge to the filaments and thus inadequate spreading.
A further problem in forming nonwoven webs by the use of a pneumatic filament distribution device is the tendency of individual filaments to become entangled with one another, thus preventing the effective separation and spreading of the filaments. Because of this tendency for filament entanglement, lower air rates are often required, which means that fewer filaments per device can be supplied to the conveyor and thus a greater number of filament distribution devices are required to form the nonwoven web.
A problem in forming nonwoven webs by the use of standard tubular distribution devices is that the pattern or "footprints" formed by the devices on the moving conveyor tend to become elongated, due to aerodynamic effects created by the proximity of adjacent devices, in the direction of conveyor movement (machine direction) causing filaments to be oriented predominantly in that direction. Nonwoven fabrics made by such processes thus tend to be weak in the direction perpendicular to the conveyor (cross direction) due to lack of orientation in that direction.
It is an object of the present invention to provide a device and method for uniformly distributing filaments from a spun filament bundle onto a moving conveyor.
It is another object of the invention to provide a filament distribution device that allows a higher air flow rate to be utilized without filament entanglement and thus a greater number of filaments per device can be uniformly distributed onto a moving conveyor.
It is also an object of the invention to provide a device and method for pneumatically separating filaments from a spun filament bundle while minimizing the amount of air that interferes with adjacent devices.
It is another object of the invention to provide a device anc method for applying an electrostatic charge to separate filaments from a spun filament bundle.
It is also an object of the invention to provide a device and method for enabling an electrostatic charge to be uniformly applied to substantially all the filaments from a spun filament bundle.
It is a further object of the invention to provide a filament distribution device having adjustable deflector plates which enables different types of spun filament bundles to be uniformly distributed onto a moving conveyor.
It is a further object of the invention to provide a filament distribution device capable of adjustable orientation with respect to the receiving conveyor to enable filament layers to be distributed at varying angles onto the conveyor.
It is a further object of the invention to provide a process for producing spunbonded nonwoven fabrics that have a machine direction to cross direction tensile strength ratio less than about 1.5.
It is a further object of the invention to provide a spunbonded nonwoven fabric that has a uniform appearance and a machine direction to cross direction tensile strength ratio less than about 1.5.
Additional objects and advantages of the invention will be set forth in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.